Mouse myeloma cells producing immunoglobulins with mutations in either their variable or constant regions have been isolated in order to study structural basis of antibody functions. Spontaneous mutations which affect antigen binding occur with a frequency of 0.1-1% and are associated with changes in idiotype and in the sequence of the heavy chain variable region of the S107 PC binding immunoglobin. In at least one case, changes in the framework resulted in a change in antigen binding. IgG2b mutant chains have been used to study the domain specificity of macrophage Fc receptors. These receptors recognize sites on the CH2 domain of gamma 2a. Finally a continuous macrophage like cell line was obtained by fusing myeloma cells to spleen cells. This cell line is sensitive to MIF.